What does it say? It says that good/correct/proper/well tuned clocks should indicate t'=t. Now, let's take another rule. We need clocks to be synchronized. What does it mean? It means that that good/correct/proper/well tuned clocks should indicate t'=t. It is not another rule, it is the same rule. And yes, you're idiots. All of you.
Galilean transformation of time